The invention concerns an X-ray analysis device comprising an X-ray source for the illumination of a sample with X-rays, a sample support for receiving the sample, and a detector for detecting the diffracted or scattered X-radiation or fluorescent X-radiation emitted by the sample, wherein an X-ray optical construction element of semi-conductor material having a plurality of channels which are essentially transparent for X-rays is provided in the path of rays between the X-ray source and the detector.
An X-ray optical construction element with the above-mentioned described features is known from U.S. Pat. No. 4,933,557.
X-ray optical construction elements in the path of rays of the X-ray analysis device may be e.g. X-ray windows, X-ray collimators or X-ray lenses. The X-ray windows have to be sufficiently transparent also for soft X-radiation. For this reason, they have been produced up to now either from elements with a small z-value or having a very small thickness. Windows for X-ray tubes (e.g. for Cu-kxcex1 rays) have been produced up to now from beryllium which has the large disadvantage that such tubes have to be disposed of as special waste, since beryllium is highly poisonous. Alternatively, also X-ray windows of CVD diamond layers have been examined which are, however, relatively expensive to produce.
It is known to use thin organic films (e.g. mylar, polypropylene etc.) as window layers for X-ray detectors, however, these X-ray windows have to be additionally supported by grid plates as support to withstand the pressure of the outer atmosphere with respect to the normally evacuated X-ray detector. From U.S. Pat. No. 5,416,821 it is e.g. known to produce such grid plates of anisotropically etched 110 silicon discs having collimating properties such that the construction element is given the combined function of an X-ray collimator window.
In contrast thereto, it is the object of the present invention to present an X-ray analysis device with the above described features, wherein one or several X-ray optical construction elements are used which are, on the one hand, not poisonous, but on the other hand are particularly transparent for X-rays, whereby it is tried to achieve a relatively high mechanical rigidity also for large openings and very short construction lengths and thus a particularly long service life and high pressure stability and density.
According to the invention, this complex object is achieved in a surprisingly simple but efficient manner in that the X-ray optical construction element comprises a semi-conductor wafer having micropores extending in the direction of the rays in an essentially parallel manner and comprising diameters of between 0.1 and 100 xcexcm, preferably 0.5 to 20 xcexcm which are formed by etching.
An X-ray optical construction element of such a semi-conductor wafer, when used e.g. as a vacuum seal, has a considerably higher density than the films of synthetic material used up to now. In contrast to X-ray windows of beryllium, such a construction component is not poisonous and can be produced with very high mechanical rigidity e.g. by silicon nitride sheets having an extremely small thickness of 50 nm, whereas beryllium windows usually have a minimum thickness of 25 xcexcm. Due to the refined structures, the inventive X-ray optical construction element can be produced with a very short construction length which mainly corresponds to the wafer thickness (100 to 700 xcexcm) whereas e.g. known X-ray collimators have minimum construction thicknesses in the range of several centimeters. Of course, the small construction thickness of the inventive X-ray optical construction element is highly advantageous also with respect to its function as X-ray window or X-ray lens, wherein there is no need to make concessions to the mechanical rigidity.
The article by V. Lehmann xe2x80x9cThe Physics of Micropore Formation in Low Doped n-Type Siliconxe2x80x9d, Journal of the Electrochemical Society, Vol. 140, No. 10, 2836-2843 (1993) discloses an electro-chemical method of producing micropores in semi-conductor wafers. The holes referred to as xe2x80x9cmacroporesxe2x80x9d in said article having diameters of a magnitude of 10 xcexcm are etched in such a manner that the walls between the generated pores are very thin (e.g. 2 xcexcm) and that each respective pore tip is closed by a likewise thin layer (in general only a few xcexcm) of silicon.
In a preferred embodiment of the inventive X-ray analysis device, the micropores of the X-ray optical construction element have not been etched continuously such that a ground surface having a thickness of between 1 to 100 xcexcm, preferably between 5 and 20 xcexcm remains. For this reason, the effective silicon layer for the penetrating X-ray light is very thin, which makes the X-ray optical construction component highly transparent. The grid of the pore walls, however, provides for a relatively high mechanical stability despite the apparently small wall thickness.
In a preferred further development of this embodiment, the inner side of the micropores is lined with a stabilizing layer which closes the micropores on one side of the semi-conductor wafer. This enhances the mechanical stability of the X-ray optical construction element without considerably impairing the transparency for X-radiation.
In order to further increase the X-ray transparency, in a particularly preferred further development, the ground surface of the semi-conductor wafer on the side on which the stabilizing layer closes the pores is etched down to the stabilizing layer. This selective etching of the wafer material leaves only a thin film of approximately 20 to 100 nm thickness of the stabilizing layer which covers the pore ground and thus generates an extremely thin window for the X-radiation. In the ideal case, the inventive X-ray optical construction element used as a window for X-ray fluorescence detectors may be transparent for energies of not more than around 100 eV.
In a particularly preferred manner, the stabilizing layer is disposed by means of CVD methods (chemical vapor deposition). CVD methods of this type are known per se. The layered material is thereby disposed onto the surface to be covered and, after cooling down, is subjected to compression thereby avoiding the formation of cracks.
It is preferred to use silicon nitride (Si3N4), boron nitride (BN), boron hydride (BH) or possibly also boron carbide or silicon carbide or even carbon as materials for the stabilizing layer. With these it is possible to produce extremely thin films which cover the ground of the pore, but still have sufficiently high mechanical rigidity and vacuum density when the inventive X-ray optical construction element is used as X-ray window.
One embodiment of the inventive X-ray analysis device is particularly preferred in which the semi-conductor wafer of the X-ray optical construction element consists of silicon. A varied and common technology for the micro-fine processing of this material is known from the production of electronic components.
The semi-conductor wafer according to the inventive X-ray optical construction element will have in general a thickness of between 10 xcexcm and 1 mm, preferably between 100 and 700 xcexcm.
It is the easiest to produce micropores having a circular cross-section with the processing methods known per se from semi-conductor technology. With modifications of the known methods, it is also possible to produce other cross-sectional shapes, e.g. elliptical cross-sections.
In the easiest case, the micropores may comprise a cross-section which is constant along the pore length. This geometrical shape will be sufficient for most cases of application.
When the inventive X-ray optical construction element is used e.g. as X-ray lens, it may be advantageous to provide the micropores with a cross-section that varies along the pore length.
One embodiment is particularly simple in which the micropore axis extends essentially perpendicularly to the surface of the semi-conductor wafer.
As an alternative, embodiments are also feasible, where in the micropore axis extends in an inclined manner to the surface of the semi-conductor wafer.
In a further alternative, the micropores are curved in their direction of passage. This requires, however, considerable more effort in production as compared with the two above described embodiments, but at the same time opens a wide field of different applications of the X-ray optical construction element according to the invention.
Thus, in a preferred further development it is possible to produce an X-ray lens which is considerably flatter in the direction of passage of the X-radiation than all hitherto known X-ray optical construction elements of this type.
In other preferred embodiments, the X-ray optical construction element has the function of an X-ray collimator which can be designed with a particularly short length due to the inventive construction of the construction element in the direction of passage of radiation.
In further particularly preferred embodiments, the X-ray optical construction element is used as an X-ray window in the X-ray analysis device. In this case, the extremely small minimum thickness of the inventive X-ray optical construction element and its very high mechanical strength are of particular advantage also with large openings and its high density is of particular advantage as vacuum seal.
One further development of the above-mentioned embodiments is particularly preferred in which the X-ray optical construction element is positioned directly in front of the X-ray detector and fulfills the combined function of a collimator window.
In this connection, it is particularly advantageous if the X-ray optical construction element forms a construction unit with the X-ray detector.
In its function as X-ray window or combined collimator window, the X-ray optical construction element may form a construction unit with the X-ray source in further embodiments.
The embodiments described above may be used also together in an X-ray analysis device according to the invention.
A preferred embodiment utilizes the particularly high rigidity properties of the X-ray optical construction element according to the invention, in which the path of rays extends partly through an evacuated space which is separated from a non-evacuated space by the X-ray optical construction element.
A further particularly preferred embodiment provides that several micropores of the X-ray optical construction element are connected in each case to form microslots through etching of the wafer material between the pores. In this manner, collimation and/or focussing of the X-radiation in a selected direction can be carried out and a better transmission through the X-ray optical construction element is achieved.
An X-ray optical construction element for the use in an X-ray analysis device of the above-mentioned kind is also within the scope of the invention, wherein the X-ray optical construction element is produced according to the following method:
(a) Application of etched cores onto the front side of a semi-conductor wafer at the locations provided for the etching of micropores by means of alkaline etching according to a lithographic standard method;
(b) Etching of the micropores from the front side of the semi-conductor wafer by means of aqueous hydrofluoric acid (HF) through application of an electric potential in the range of several Volts, wherein the etching flow is generated preferably by illumination of the rear side of the semi-conductor wafer;
(c) Coating the semi-conductor wafer by means of the CVD method, preferably with Si3N4;
(d) Etching of the rear side of the semi-conductor wafer down to the CVD layer in the tips of the micropores.
Finally, in further advantageous embodiments the following additional step may follow:
(e) Removing of the CVD layer, preferably by means of hydrofluoric acid.
The coating which served as mechanical support during the production of the X-ray optical construction element is e.g. no longer absolutely necessary for the function of the component as collimator such that its removal results in a particularly high transparency for the X-radiation. However, the mechanical stability is then no longer sufficient to be used as X-ray window with pressures of relatively high differences.
Further advantages of the invention can be gathered from the description and the drawing. The features mentioned above and below may be used in accordance with the invention either individually or collectively in any arbitrary combination. The embodiments shown and described are not to be understood as exhaustive enumeration but rather have exemplary character for describing the invention.
The invention is shown in the drawing and is explained in more detail by means of embodiments. In the drawing: